Resins obtained when curable resin compositions harden are known to have superior electrical characteristics pertaining to dielectric characteristics, low volume resistivity and insulating strength. In addition, these resins have superior mechanical properties including bending strength, compression strength and shock strength. These resins are used as adhesives and sealing resins for electrical and electronic elements and as FRP resins. The resins obtained by curing curable resin compositions are generally rigid and have poor flexibility. In addition, because the setting shrinkage ratio is large, stresses are placed on the electrical or electronic elements when the resin is used as a sealing resin for electrical or electronic elements. Consequently, cracking occurs in the hardened resin, or the electrical or electronic element cracks, or spaces develop between the hardened resin and the electrical or electronic element due to poor adhesion between the hardened resin and the electrical or electronic element. In addition, curable resin compositions form hardened resins that have large thermal expansion coefficients in comparison to the thermal expansion coefficients of electrical or electronic elements. Thus, when the resin-sealed electrical or electronic elements undergo a heat cycle, large stresses are placed on the electrical or electronic elements, and cracking occurs in the hardened resin, or spaces arise between the hardened resin and the electrical or electronic elements due to poor adhesion between the hardened resin and the electrical or electronic elements. Moreover, there have been problems with deterioration of the resin-sealed electrical or electronic elements due to ingress of water through cracks produced by shrinkage of the curable resin composition during setting, by expansion and shrinking due to heating, or due to the development of spaces between the hardened resin and the electrical or electronic elements.
Thus, various curable resin compositions have been offered whereby the flexibility of the resin is improved in order to lessen the stresses that occur during setting and whereby adhesion of the hardened resin with the electrical or electronic element is improved. Examples of compositions that have been offered include a curable resin composition comprised of a catalyst for curing, an inorganic filler, an organopolysiloxane composed of trifunctional siloxane units (T units) and bifunctional siloxane units (D units) having epoxy groups, methylphenylpolysiloxane and an epoxy resin as disclosed in Japanese Kokai Patent Application No. Sho 56[1981]-145942. Another curable resin composition is comprised of an inorganic filler, a curing agent, an organopolysiloxane composed of bifunctional siloxane units (D units), trifunctional siloxane units (T units) and monofunctional siloxane units (M units) having at least one each of an amino or epoxy group and a hydroxy group or a group that can undergo hydrolysis, and an epoxy resin as disclosed in Japanese Kokai Patent Application No. Sho 56[1981]-136816. Japanese Kokai Patent Application No. Sho 60[1985]-58425 discloses a curable resin composition comprised of a dimethylpolysiloxane having molecular side-chain epoxy groups and polyoxyalkylene groups, a phenol resin and an epoxy resin. Finally U.S. Pat. No. 4,880,882 discloses a curable resin compositions comprised of a curable resin and spherical particles of silicone rubber. However, the curable resin compositions described in Japanese Kokai Patent Application Nos. Sho 56[1981]-145942 and Sho 56[1981]-136816 necessarily use organopolysiloxanes as the bifunctional siloxane unit (D unit), and thus there is the disadvantage that the glass transition point (Tg) of the resulting hardened resin decreases, this decrease being particularly dramatic when an organopolysiloxane is used that has epoxy groups and is composed of (CH.sub.3).sub.2 SiO.sub.2/2 units (D units). The curable resin compositions offered in Japanese Kokai Patent Application No. Sho 60[1985]-58425 use dimethylpolysiloxanes having molecular side-chain epoxy groups and polyoxyalkylene groups, with the result that the water absorption of the resulting hardened resin is considerable and the resin is inappropriate for use as a sealing resin for electrical and electronic elements. Moreover, although the hardened resins obtained from the curable resin compositions offered in U.S. Pat. No. 4,880,882 have superior flexibility, their flowability is poor, and thus have inferior working properties as a resin seal for electrical and electronic elements. Moreover, these hardened resins have inferior adhesion with respect to electrical and electronic elements.
The object of the present invention is a curable resin composition that has superior flowability prior to setting and produces a hardened resin with superior flexibility and adhesion after setting.